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Surgery with sound waves?

Using loudspeakers to manipulate objects in mid-air could lead to scalpel-free surgery. Viviane Richter explains.

A levitating polystyrene bead held in place by sound waves (which are depicted in a computer-generated overlay). – Image courtesy of Asier Marzo, Bruce Drinkwater and Sriram Subramanian

Could sound waves one day replace scalpels in operating theatres? Using a small panel of loudspeakers, Asier Marzo and his colleagues at the University of Bristol have generated a pattern of sound waves capable of lifting a polystyrene bead and moving it around in mid-air as though by an invisible pair of surgeon's tweezers. They published the work in Nature Communications in October.

“It’s been known to be theoretically possible,” says Rick Weber, a physicist at Argonne National Laboratory’s Advanced Photon Source in Chicago, but “this works”. Marzo has “very elegantly reduced [it] to practice”.

Sound waves ripple through air (and other materials) like a Mexican wave travelling around a sports stadium, causing the air molecules to vibrate in turn. These vibrations can pack a punch – you might have felt the puffs of air hitting your face or the ground shaking beneath your feet after venturing too close to the speakers at a concert.

When sound waves crash into each other they either cancel each other out or add up to create a bigger, louder wave that carries more force.

It’s a neat trick, but could anything useful be done with it?

Previous sound wave experiments have used carefully targeted patches of these higher amplitude sound waves to lift small objects – including spiders, which remained unharmed. But they could only do this by surrounding the object with speakers to ensure that different waves hit the same spot at the same time.

To get around that limitation, the Marzo team manipulated the timing of sound waves from a panel of 64 one-centimetre-square speakers to control how and where waves added up.

By slightly off-setting the timing of the different waves, the team created two patches of higher amplitude sound waves, which they used like an invisible pair of tweezers to pick up and rotate a two-millimetre polystyrene bead. They could even trap the bead inside an invisible ‘bottle’ made of sound waves.

It’s a neat trick, but could anything useful be done with it?

One potential use is handling sterile samples without touching them. But Marzo sees most potential in non-invasive surgery – to use the sound waves to move drug capsules around the body for release at a specific location, for example. High-intensity focused ultrasound is already used to treat some kidney and prostate cancers, he points out, although in this case the waves are focused to a single, high-energy spot to destroy cells rather than to move and manipulate them.

Marzo is already testing his system’s ability to move objects in water, rather than air, as a step towards using it on the human body. He next plans to adapt a high-intensity focused ultrasound machine, which has been designed to create more powerful waves to go through tissue, and for which the human safety limits are already known.

“We’ve showed that it’s possible,” he says. “But the real breakthrough will be in a medical application.”

Sound waves projected from a flat surface can exert considerable forces on objects trapped between them. The pattern of sound waves can be updated in real-time to move the trapped object around, giving unprededented control over the object being manipulated. Credit: Video courtesy of Asier Marzo, Matt Sutton, Bruce Drinkwater and Sriram Subramanian